This invention relates to a process for the preparation of lower alkyl esters of higher fatty acids having improved properties for processing into surface-active .alpha.-sulfo-fatty acid esters.
The preparation of surface-active .alpha.-sulfo-fatty acid esters from fats and oils especially of natural origin, has been known for years. For example, U.S. Pat. No. 2,195,187 describes .alpha.-sulfo-fatty acids and their esters as surface-active compounds or surfactants. They are obtained by the sulfonation of lower alkyl esters of saturated higher fatty acids with sulfur trioxide. The lower alkyl esters of higher fatty acids are obtained by reesterification of hydrogenated fats or oils with monovalent, lower alkanols, especially methanol, or by the cleavage of glycerides and subsequent esterification of the fatty acids.
Henkel KGaA has been concerned intensively with this class of surface-active .alpha.-sulfonated fatty acids and respective fatty acid esters as well as their salts. The German Published Application DE-AS No. 12 46 718, for example, describes a process for the preparation of this class of compounds. Fatty acids and fatty acid esters that have 6 to 28 carbon atoms in the fatty acid radical, have no other groups that can be sulfonated or sulfated beside the carbon atom in .alpha.-position, and have an iodine number of less than 5, are sulfonated with a mixture of sulfur trioxide and inert gas, and the reaction product is neutralized. A parallel procedure working with the same substances is described in DE-AS No. 12 48 645.
One of the main problems of the area involved here is the color instability of the lower alkyl esters of higher fatty acids in the sulfonation step. The crude products obtained are dark colored, brown-black and must be processed into light-colored products for use in washing and cleaning agent compositions. While the color of the crude sulfonation products does depend to some degree on the working conditions, the technical utilization of these, of themselves interesting possible starting materials, still is hampered by the following principle: The higher the yield of the reaction in the sulfonation step (the degree of sulfonation), the darker the color of the reaction product and the greater the problems to obtain light-colored end products.
The significance of the constitution of the fatty acids or the fatty acid mixtures to be sulfonated is considered as certain knowledge by the experts in the field. Especially the stipulation is made that the fatty acids to be sulfonated in the .alpha.-position shall not have any double bonds, or as few as possible, nor any other types of reactive groups, especially hydroxyl groups. With the selection of suitable fats or oils, this problem is limited to the removal of unsaturated bonds in the fatty acid molecule. These sources of interference are eliminated by hydrogenating the starting material as completely as possible before the sulfonation. The literature of the state of the art indicates required iodine numbers of less than 5, preferably less than 2. Much lower iodine numbers, that is, those in the range from 0.1 to 0.3, are used in the practical examples.
The removal of interfering accompanying substances by distillation or other methods from the fatty acids or fatty acid mixtures to be sulfonated is called for to reduce the discoloration problem, for example in DE-AS No. 12 48 645.
However, bleaching of the crude sulfonic acid derivatives always is necessary as final step in the process. The state of the art has evolved particularly two types of procedures for this purpose: Acid bleaching with hydrogen peroxide, as described in German Patent DE-PS No. 11 79 931, or combination bleaching in which the initial hydrogen peroxide bleaching is followed by the neutralization of the sulfonated and partially bleached material, after which a final bleaching step is conducted, again with hydrogen peroxide or, more advantageously, with hypochlorite, as described in DE-AS No. 12 34 709, for example.
The problem of discoloration becomes particularly difficult when the sulfonation is to be increased to yields exceeding 90%, or even to degrees of sulfonation above 95%. The teachings of the DE-OS No. 14 43 995 concern themselves with the problems arising from this. According to information in this published patent specification, sulfur trioxide has a strongly decomposing effect on saturated fatty acid esters free of alcoholic hydroxyl groups, and this results unavoidably in very dark, discolored sulfonation products in the preparation of highly sulfonated products with a degree of sulfonation of at least 90%, preferably 94%, and especially at least 96%.
DE-OS No. 14 43 995 recommends the addition of water to the sulfonation or the sulfonation product in order to keep the discoloration within limits. But this method generates new problems for the practical application in addition to observing certain temperatures. The viscosity of the sulfonated starting product in the strongly acid range is seriously affected by smallest amounts of water. Even the addition of 2% hydrogen peroxide in the form of a 35% solution causes a steep rise in viscosity with sulfonation products having a chain length of C.sub.16 /C.sub.18. The danger of clogged lines can happen in the continuous technical-scale process.
According to the available experience, the various difficulties developing in different steps of the entire process force a compromise between the degree of sulfonation and bleaching. The optimal degree of sulfonation obtainable in practice are close to approximately 90%.
It is common knowledge that natural starting materials such as plant and/or animal fats or oils generally are subject to certain variations in quality, due, for example, to the peculiarities of their origin and/or their handling up to the planned further processing. These variations bring with them a certain increase in problems in further processing, in contrast to standardized, purely synthetic starting materials. Consequently, there exists in the respectively more circumscribed field of the lower alkyl esters of higher fatty acids, a need for the standardization of the product quality of different starting materials by a simple refining procedure as well as for improvement of the processing characteristics of the refined materials, which guarantees dependable and improved subsequent processing results and/or products with improved characteristics in the large-scale operation.